Transitions and predictions

BioLogos has re-posted a video on transitional fossils from 2011, which would have been more useful if it had tackled some of the doubts many have had over the incompleteness of the fossil record since Darwin’s time. Instead it set up a Creationist strawman in the form of Kirk Cameron and his notorious crocoduck. It’s hard to believe Cameron ever took his hybrid animals seriously, but if he did it’s certainly a comment on the lack of intellectual sharpness amongst some US Fundamentalists – but we knew that already.

It’s also obvious that his arguments are not those which have been discussed by serious critics of Darwinian evolution, and particularly of classic gradualism, over the years. I doubt that even many thinking Creationists really believe evolution posits random chimaeras of modern creatures. And no serious critic is unaware of the branching nature of evolutionary lines. Many are even more aware of their reconvergence by HGT than some Darwinians seem to be.
Setting up such an Aunt Sally, and giving simplified replies to it, will I suspect not so much win over the target audience of suspicious conservative evangelicals  as alienate them. And that’s before veteran BL poster melanogaster started insulting them gratuitously (together with those who are neither evangelicals nor Creationists). One wonders also why the BioLogos article presented Australopithecus sediba as a known transition between australapithecines and Homo when that is still seriously disputed. As BioLogos‘ own Jim Kedder recently wrote:

It has also been suggested that there are post-cranial elements in the hip and leg bones that align it with later Homo, although, at this point, the evidence for this position is scant.

To many, including me, what is in question is not the overall trend of the development of life on earth, but the sufficiency of the mechanisms proposed to explain it. In the video Christian palaeontologist Keith Miller, commenting on the sketchy nature of the fossil record, wisely says that it cannot give a complete picture, but is sufficient to show “a pattern of change over time.” I wouldn’t argue with that, but neither would many Creationists.

Donald Prothero, some years ago, made what seems a generally accepted estimate of some 250,000 described fossil species, which he placed at about 5% of the number of presently estimated living species. He compared the two figures to show the paucity of the fossil record, both in terms of the large number of species ill-suited to fossilisation, and the contingency of the circumstances leading to fossilisation. His conclusion was that we must possess only a tiny fracton of the total number of extinct species. But actually, one could equally explain it by the Creationist argument that only a small number of species have died out, and that there never were many more than there are today, or by a guided evolution model in which transitions were large and purposeful.

A more informative comparison, it seems to me, is that between the number of species and the total number of individual fossils identified. This is clearly a very approximate estimation, but one can at least get into the right ballpark. Graeme Lloyd of London’s Natural History Museum says that most large museums will have a collection of several million fossils. Presumably they will have been labelled and classified, at least provisionally. Wikipedia lists several hundred natural history museums, of varying sizes, across the world, so we might conservatively guesstimate that there are upwards of 200 million scientifically  identified fossils in them. Some of those will actually be misidentified new species – but equally some will be wrongly differentiated.

What that means is that, on average, there are maybe 80+ specimens of each identified fossil species. If one assumes that (a) Darwinian gradualism usually operates and (b) that fossilisation is extremely rare and fortuitous, then that finding is unexpected. One would predict it would be rare to find any two fossils of the same species, as a constantly evolving population was sampled in preservation. Yet in some cases, there are thick fossil beds consisting of millions of accumulated creatures like ammonites of the same species, as we have on my nearby Jurassic coast. But even large vertebrates also tend to be identified by multiple examples.

There are, for instance, around a dozen examples of Archaeopteryx, whose exact relationship to the birds and dinosaurs is still uncertain, because we don’t have a scatter of closely related genera. That is easy to explain, as all the specimens come from one local rock formation, where conditions for fossilisation were unusually good. So the “incomplete fossil record” argument makes complete sense there.

But there are also about 30 Tyrannosaurus rex finds, and they were scattered all across western North America. Iguanadon is known from “large numbers” of specimens across Eurasia and North Africa, and all are now thought to belong to one or maybe two species.

Bob Bakker, one of the foremost dinosaur palaeontologists, was confident enough of the statistical sampling in North America to use comparative counts of herbivore and carnivore fossils in the collections to build his theory of theropod warm-bloodedness. If fossilisation were as fortuitous as is claimed (ie assuming that perhaps 97.75% of species are missing from the record), that calculation should never have passed peer review. But in fact it strongly suggests that the steady gradualism Darwin originally  proposed either has no basis in fact whatsoever, or is so rare as to be an anomaly. The stasis/saltation pattern described by Gould remains far more likely.

The same argument is true even in the finest examples of evolutionary transitions, like that of the horse, at which the BioLogos video hints by background graphics without actual comment – maybe just to annoy Creationists, who like to deny its validity. I, on the other hand, don’t deny the truth of the general scheme, described in quite a succinct yet full summary on Talk Origins, complete with obligatory swipes at Creationists . Here is as branching and non-linear a tree as one could wish, as the author points out, consisting of some 30 genera and rather more species living over more than 50 million years.

But again, what is the “sampling rate” here? Has the fossil record left incredibly rare snapshots of a constantly changing evolutionary scene? Or is the relatively small number of species found closer to the total that ever lived? Once again, some at least of the evidence seems to suggest the latter. The “root” genus of the equine tree, Hyracotherium, has only one known species, but it is described from “hundreds of specimens across the northern hemisphere”. Is the fossilising process a lot less fortuitous than we are led to believe, or did Hyracotherium alone, of all the many similar forms, practise mummification?

Modern evolutionary theory is not blind to all this, of course. It is proposed that change of habitat and/or isolation leads a small population from a relatively stable species to evolve in a geologically short time, of a million years or so, before the successful new species explodes across the world. Yes, maybe. But as the much examined chimp-human divergence shows, that’s an awful lot of trial and error to pack into a rather non-Darwinian time-frame. And it also means that, once again, as Niles Eldredge said, “evolution always seems to happen somewhere else.”